A circuit pattern composed of electrodes, resistors, etc., on which various electronic parts are mounted, has been generally formed on a ceramic base made of alumina, etc., and the electrodes are generally formed by screen printing a paste comprising a noble metal, e.g., silver or a silver-palladium alloy, on the ceramic base followed by baking in air.
However, because not only of expensiveness of the noble metal paste but also of the demand for sufficient migration resistance of the resistive paste to cope with the decreasing electrode distance due to size reduction of electronic equipment and parts, the noble metal paste has recently been displaced with a paste of a base metal, e.g., copper, nickel or aluminum. Such a base metal paste is screen-printed on a ceramic base and baked in a neutral or reducing atmosphere to form an inexpensive electrode pattern having excellent characteristics.
When electrodes are formed by using such a base metal paste, resistors which are arranged to bridge over the electrodes should also be formed by using a resistive paste which can be baked in a neutral or reducing atmosphere. Examples of known resistive pastes which can be baked in a neutral or reducing atmosphere include LaB.sub.6 -based pastes (as described in JP-B-59-6481, the term "JP-B" as used herein means an "examined published Japanese patent application"), NbB.sub.2 -based pastes (as described in JP-A-63-22430, the term "JP-A" as used herein means an "unexamined published Japanese patent application"), and Nb-La-B-based pastes (as described in JP-A-2-249203).
A desired surface resistivity over a broad range has been obtained by varying the mixing ratio of a resistive material and glass frit. In using the LaB.sub.6 -based or NbB.sub.2 -based resistive pastes, however, the surface resistivity suffers drastic changes with a slight variation in glass frit amount due to poor affinity between the resistive material and glass frit. Therefore, the range of surface resistivity in which satisfactory reproducibility can be assured has been limited.
On the other hand, resistors formed of the Nb.sub.x La.sub.1-x B.sub.6-4x -based paste show a milder increase in surface resistivity than with those formed of the LaB.sub.6 -based pastes or NbB.sub.2 -based pastes. Accordingly, the Nb.sub.x La.sub.1-x B.sub.6-4 4-based paste has an advantage of a broadened surface resistivity range of from 10 .OMEGA./square to 10 M.OMEGA./square by varying the mixing ratio of resistive material to glass frit. However, the resistors formed of the Nb.sub.x La.sub.1-x B.sub.6-4x -based paste, particularly those adjusted to have a low surface resistivity (e.g., from about 10 .OMEGA./square to 100 .OMEGA./square), show a tendency of the temperature coefficient of resistivity (hereinafter abbreviated as "TCR") shifting to the plus (+) direction with its absolute value getting far from zero. In this point, they do not always satisfy the characteristics required for practical use.